The present invention relates generally to artificial cardiac pacemakers, and more particularly to implantable cardiac pacemakers in which the pacemaker pulse generator case forms an anodal surface.
Cardiac pacemaker therapy has passed through several important developmental stages, including those which have increased the longevity of the devices, decreased their size, and made their functions more flexible. Improvements have extended also to freeing the devices of interference from external noise sources, which allows enhanced recognition of the intrinsic cardiac signals.
A common technique for reducing interference from external noise sources involves the use of bipolar electrodes, which enables sensing of the intrinsic heart rate between a pair of points in the heart. The magnitude of the average intrinsic rate ranges from 1.0 to 8.0 millivolts (mv). However, bipolar electrodes are not without disadvantages, primarily attributable to the existence of two electrical channels, which affect the fracture rate, ease of handling, and thickness of the electrode lead.
Unipolar electrodes have a thinner lead diameter, greater flexibility, and fewer complications than the bipolar versions, and find greater acceptance, especially in Europe. The unipolar electrodes employ an endocardial electrode positioned in the heart as one electrical pole, which is connected via an insulated helical conductor to the pacemaker circuitry and thence, to the implanted pulse generator case which acts as the second electrical pole. During stimulation of the heart, the case acts as the anode, and the endocardial electrode is the cathode.
In the infancy, of implantable cardiac pacemaker therapy, the pulse generator case was composed of metal such as stainless steel, titanium or alloys thereof, without any coating so that the case was completely electrically conductive.
A disadvantage of unipolar electrode systems, in addition to the greater antenna effect between the generator case and the endocardial electrode tip, is that muscle potential can produce an undesired effect. As a practical matter, flexation of the pectoral muscle, on which the pulse generator is typically implanted in the patient, can produce voltages of similar amplitude and frequency to the intrinsic cardiac signals. This interference is conducted through the pulse generator case, with a resulting undesirable and potentially dangerous influence on the pacemaker functions. The effects may include improper inhibition, and, in the dual chamber pacemakers, improper triggering and initialization of re-entry tachycardias.
Customarily, muscle produced improper triggering of the pacemaker is avoided by coating the entire pulse generator electrically conductive case with an electrically insulating material except for a small uncoated window which serves as the anodal electrode contact. The coating material is typically a thermoplastic polymer film known commercially as parylene, which is both biocompatible and an excellent electrical insulator. In the usual procedure, the posterior side of the case, meaning all parts facing the (inside) pectoral muscle, all side walls, and part of the anterior (frontal) side of the case are coated with parylene, and only a small part of the anterior side of the case is the anodal window that faces the (outside) fatty tissue.
This method greatly reduces muscle-induced interference, and has been clinically proven and implemented in many thousands of pacemakers.
A further advantage of the coating technique is that it avoids inappropriate stimulation of the pectoral muscle by pacing signals between the generator case and the endocardial electrode tip, and thus eliminates the otherwise annoying twitch of the pectoral muscle with each stimulation of the myocardial tissue.
A disadvantage of such a coating method is that the pulse generator cannot be inverted or "flipped" (i.e., turned over so that the normally posterior side of the case is facing front, toward the fatty tissue, and the anodal window is facing inward, toward the pectoral muscle, with header up as in the "normal" orientation) for implantation in the patient. Only by allowing such inversion can either the right side or the left side of the case be selected as the outlet for the connector and lead, with the header up.
This becomes increasingly significant with diminished size of pulse generators, and the standard 60 centimeter (cm) length of the electrode lead which must be sufficiently rolled up to fit the subcutaneous pocket formed in the patient for implantation of the generator. Standard pulse generators which allow the possibility of an electrode lead outlet on only one side often cause mechanical difficulties for the patient. it would be desirable to have the capability to conveniently implant the generator with the case inverted, i.e., turned over, in some patients so that the electrode lead exits the other side. This capability would be desirable in clinical practice not only to accommodate right or left pectoral implantation but in those instances where the device must be positioned behind the pectoral muscle.
It is a principal object of the present invention to provide a pulse generator with an electrically insulating coating covering all but a portion of the electrically conductive case, that allows for this freedom of positioning choice.